The invention relates to a cast turbine blading unit having an airfoil and a platform region.
An impingement cooling system for a gas turbine blading unit is known from DE 26 28 807 A1. The gas turbine blading unit is directed along a blading unit axis and has an airfoil and a platform region along the blading unit axis. In the platform region, a platform extends radially away from the airfoil and transverse to the blading unit axis. Such a platform forms part of a flow duct for a working fluid which flows through a gas turbine in which the turbine blading unit is installed. In a gas turbine, very high temperatures occur in this flow duct. Because of this, the surface of the platform subjected to the hot gas is subjected to severe thermal loading. In order to cool the platform, a perforated wall element is arranged in front of the surface of the platform facing away from the hot gas. Cooling air enters via the holes in the wall element and impinges on the surface of the platform facing away from the hot gas. Efficient impingement cooling is achieved by this means.
GB-PS 1 289 435 relates to guide elements, in particular gas turbine blading units, for gas flows. A guide element which is built up in laminar fashion and which can be cooled by transpiration cooling is arranged on a cast component. This construction cannot be applied to cast turbine blading units.
DE 26 43 049 A1 shows a cooling arrangement for cooling the platform of a turbine blading unit. As compared with the arrangement in DE 26 28 807 A1, mentioned above, a plate with openings is arranged in front of the surface of the platform facing away from the hot gas surface and cooling air flows through these openings towards the platform.
The object of the invention is to provide a cast turbine blading unit which can be subjected to high thermal loading and in which only small thermal stresses occur in the platform region. In accordance with the invention, this object is achieved by means of a cast turbine blading unit directed along a blading unit axis and having an airfoil and a platform region in sequence along the blading unit axis, the platform region comprising a hot gas platform extending transverse to the blading unit axis and bounding the airfoil and comprising a load-carrying platform opposite to the hot gas platform, the load-carrying platform being designed to accept forces which may be caused by a working fluid flowing around the airfoil.
A turbine blading unit is secured in the turbine, in particular on the turbine casing, by means of the platform region. In consequence, the platform must accept loads which are caused by forces acting on the airfoil. Such forces are caused by the pressure of the hot working fluid, for example a hot gas or steam, flowing through the turbine. The acceptance of these loads demands that the platform should have a minimum thickness so that it can transfer the forces to the turbine casing without deformation. At the same time, the platform as described above must bound the flow duct through which a hot gas flows. The invention pursues a new path in the design of the platform region for cast turbine blading units in that the platform region is configured as a double platform made up of two opposing platforms. This achieves the effect that the hot gas platform, which bounds the flow duct and is subjected to the hot gas, can be given a thin configuration. The configuration in two platforms provides a division of functions for the platforms. The hot gas platform is substantially responsible for bounding the flow duct and, therefore, for channeling the hot gas. The opposing load-carrying platform, which is not subjected to the hot gas, accepts the loads caused by the forces acting on the airfoil. This separation of functions makes it possible to provide the hot gas platform with such a thin configuration that the hot gas channeling is ensured without the need to accept substantial forces. The thin configuration of the hot gas platform gained in this manner provides the particular advantage that comparatively small thermal stresses arise in the hot gas platform. The configuration of the platform region as a double platform is also advantageous when compared with configurations in which a single-piece platform is reinforced by ribs on the surface facing away from the hot gas. This is because high thermal stresses can likewise occur at the transition locations between the ribs and the platform.
The hot gas platform is preferably substantially thinner than the load-carrying platform. Because the hot gas platform only has to accept, at most, a comparatively small part of the loads occurring, it can be provided with a thinner configuration than the load-carrying platform. The load-carrying platform accepts the major proportion of the forces occurring.
The airfoil is part of a profile section extending through the platform region, the hot gas platform and the load-carrying platform each having an inner edge by means of which they are connected to the profile section. In addition, they each have an outer edge by means of which they are connected to one another. It is also preferable for the hot gas platform and the load-carrying platform to be connected to one another only by means of their respective inner edges and their respective outer edges. This provides a small connecting surface between the hot gas platform and the load-carrying platform. Because of this small connecting surface and because of the connection via the respectively outer edges, only small thermal stresses occur in the double platform configuration for a high level of mechanical stability. Thermal expansions can take place relatively freely due to the small number of connection locations.
Guide elements for guiding a cooling medium to the hot gas platform are preferably arranged between the hot gas platform and the load-carrying platform. Such guide elements can, for example, be panels which sub-divide the space between the platforms in the manner of chambers or can also, for example, be ducts directed vertically between the platforms. By means of such guide elements, a cooling mediumxe2x80x94in particular cooling airxe2x80x94can be efficiently directed against the surface of the hot gas platform facing away from the hot gas. This, in particular, permits efficient impingement cooling.
The guide elements are preferably configured with a wall thickness which is thin relative to the hot gas platform. Because of the thin configuration of the guide elements, no substantial additional thermal stresses are caused.
The load-carrying platform preferably has a plurality of through holes directed towards the hot gas platform. A cooling medium, in particular cooling air from a compressor of a gas turbine, can flow through the load-carrying platform and towards the hot gas platform, thus efficiently cooling the latter.
The turbine blading unit is preferably configured as a gas turbine blading unit, in particular for a stationary gas turbine.
The invention is described in more detail by means of the drawing. In this: